The present disclosure relates to controlling a transformerless uninterruptible power supply (UPS) system.
An uninterruptible power supply (UPS) is an electrical device that can supply power to a load despite variations in quality and/or availability of utility-based power. Some transformerless UPS systems may employ a neutral-point-clamped (NPC) inverter, flying capacitor inverter, and/or cascaded H-bridge inverter. Some of these converters, such as the NPC inverter, may be quite efficient, but could become unstable when a steady-state imbalance in the neutral-point voltage arises. As a result of a neutral-point imbalance, output voltages could become distorted with a DC offset in the output voltage and/or current. Such output DC offsets could, in turn, cause devices to fail due to higher stress, saturate filter inductors, and/or deteriorate the output power quality. Although various strategies have been introduced to solve this problem, these solutions may be costly and/or highly complex. Many of the existing solutions are based on adding a zero-sequence voltage component to the output voltage and/or by manipulating the redundant switching vectors when applying vector control. Some of these techniques may also address the so-called short-pulse problem. Most of these methods, however, have disadvantages in terms of the maximum usable modulation index. Additionally, these methods may produce an increase in the switching losses of the NPC inverter and may require extremely complex control algorithms.